Bats Global Markets Inc.’s merger with Direct Edge Holdings LLC will put a broker-owned stock exchange operator in the top ranks on Wall Street for the first time since NYSE Euronext and Nasdaq OMX Group Inc. went public.

Securities regulators fined Knight Capital Americas LLC $12 million for the trading malfunction that roiled the U.S. stock market in August 2012, saying the firm ignored dozens of error messages before its computers bombarded exchanges with millions of unintended orders.

Thomas Joyce, who built Knight Capital Group Inc. into one of the biggest U.S. market making firms only to see it driven to the brink of bankruptcy and then into a suitor’s arms, is leaving the company.